RFR: 8320969: Shenandoah: Enforce stable number of GC workers

[Aleksey Shipilev]

See bug for rationale.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`
 - [x] Linux x86_64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`
 - [x] Linux AArch64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`

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Commit messages:
 - No dynamic GC workers

Changes: https://git.openjdk.org/jdk/pull/16878/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16878&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320969
  Stats: 206 lines in 5 files changed: 10 ins; 175 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/16878.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16878/head:pull/16878

PR: https://git.openjdk.org/jdk/pull/16878